This invention relates to techniques for generating and detecting ultrasonic waves and, more particularly, to electromagnetic acoustic transducers.
An increasing emphasis on efficiency and economy in many areas of modern structural design has stimulated a more widespread use of the techniques of nondestructive testing. Nondestructive methods are important because of their ability to locate a structural defect at an early stage in the life of a flaw, permitting the appropriate corrective action, such as removing and replacing the defective component, to be initiated before the defect causes a catastrophic failure.
Before nondestructive testing methods became available, it was necessary to design structural components under the assumption that flaws of a certain size would be found in the construction materials. This assumption led to specifying components of sufficient size and strength to function properly even when the assumed defects were present. When nondestructive testing measures are implemented, however, such structural components may be manufactured and assembled more economically by reducing dimensions and substituting less expensive materials. Nondestructive inspection techniques can thus be utilized to maintain a desired level of reliability in a physical structure while reducing construction and material costs.
One of the many types of nondestructive testing techniques is ultrasonics, in which the interaction between acoustic wave energy and the internal structure of an object is analyzed to predict the physical integrity of the object. A key element in any ultrasonic nondestructive testing system is the transducer, which is used to convert electrical energy into acoustic wave energy and vice versa. Traditionally, the high conversion efficiency and modest cost of piezoelectric materials have led to their widespread use as ultrasonic transducers in many applications. Piezoelectric transducers are hampered, however, by the need to be coupled to the ultrasonic medium by a liquid or solid bond.
The ability to operate at high speeds and elevated temperatures, in remote locations, with broadband and reproducible acoustic coupling, and without the need for subsequent clean up operations of a liquid bond have spurred the development of noncontact techniques, such as electrostatic transducers, optical techniques, and electromagnetic transducers, which have supplanted piezoelectric transducers in many applications. One of the most promising of the noncontact transducers is the electromagnetic acoustic transducer (EMAT). In general, an EMAT consists of a coil of wire which is positioned within a static magnetic field near the surface of a conducting material. When an RF current is applied to the coil, eddy currents are induced in the material. Lorentz forces are exerted on the eddy currents as a result of the magnetic field and are transmitted to the lattice structure of the material to generate an ultrasonic wave. A reduction in inspection time, an ability to operate in remote and inaccessible locations, and a reduction in transducer wear are all significant economic advantages which are offered by an EMAT-based nondestructive testing system.
EMATs may be fabricated with a number of different coil and magnet configurations to suit the requirements of a particular application. U.S. Pat. Nos. 3,850,028; 4,048,847; 4,080,836; 4,092,868; 4,104,922; 4,127,035; 4,184,374; 4,218,924; 4,232,557; and 4,248,092, for example, illustrate the variety of approaches which are available. While EMATs have thus been utilized in many nondestructive testing situations, the problem of exciting a broadband ultrasonic signal has presented a significant limitation in the past, since the narrowband operation traditionally employed limits resolution and consequently either restricts the operation of EMATs to parts of very simple geometry or requires extensive signal processing to separate flaw information from boundary information.
Early experiments showed that broadband operation could be achieved with relative ease at frequencies of a few MHz. At higher frequencies, however, broadband transduction becomes considerably more difficult because the inductance of the EMAT coil limits the rise time of the exciting current pulse. Thus, a new approach which would render an EMAT capable of efficiently generating and detecting ultrasonic wave energy over a wide range of frequencies would satisfy a long felt need in this area of technology.